[nxxxsooo]

Implements the colors half of #58, building on the theme.rs palette from #85 as suggested there.

[colors]
heading = "#89b4fa"
accent = "light-blue"
block = "236"
selection = "#45475a"

• [colors] in config.toml: a role replaces that colour in whichever palette is in force (light or dark, after PLANNOTATOR_TUI_THEME, [ui] theme and detection have picked one). Roles left out keep today's values, so an empty table changes nothing.
• Roles: accent, heading, comment, looks-good, delete, comment-bg, looks-good-bg, block, toolbar, cursor, selection. block covers the focused block and the focused tree row, per @alesch's notes.
• Colour strings are ratatui's: names (yellow, light-cyan), indexes (238), #rrggbb.
• Strict: an unknown role or an unreadable colour is an error that names it (unknown color "chartreuse"; expected a name like yellow, an index 0-255, or #rrggbb).
• plannotator-tui config prints the roles that are set.
• README: a [colors] block listing every role with its dark default.

Shape

• Palette gains the foreground roles that were inline Color::Cyan / Yellow / Green / Red constants in draw.rs, layout.rs, header.rs, menu.rs, pick.rs and archive_view.rs. Both palettes share them, since ANSI colours already follow the terminal.
• Palette::with(&Colors) applies overrides; install stores the result in the OnceLock (an owned Palette now, not &'static).
• run_ui loads the config once for both theme and colours. A config that fails to parse still falls back to defaults, as before.

For review

• heading sets every level. Today h1/h2 are Cyan and h3+ LightCyan, so internally they're heading/subheading; I left subheading out of the config to keep the role list small. Happy to expose it.
• A configured cursor or selection is a plain background. The dark selection's REVERSED and the light cursor's white glyph would otherwise fight a colour picked to match a theme.
• The header's send/sent/blocked buttons set their own fg and bg, so I left them alone.
• This touches config.rs and cli.rs, as feat(keys): rebind every key under [keys] #104 does. The overlap is small: one field each, and the run_ui config load.

Motivation: I maintain an OpenCode plugin that opens plannotator-tui for /annotate. It already passes PLANNOTATOR_TUI_THEME from OpenCode's light/dark mode. With [colors] and PLANNOTATOR_TUI_CONFIG it can pass the active OpenCode theme's colours too.

Testing: cargo test --workspace (320 pass; new unit tests in theme.rs and config.rs, plus an integration test that config prints the set roles), cargo clippy --workspace --all-targets clean, cargo fmt --all --check. Checked by hand in tmux: headings at every level and the focused block render in the configured truecolor values.

[backnotprop]

Thanks @nxxxsooo, nice work. I rebased it onto [keys] and pushed a small follow-up: a bare index like heading = 238 now parses, plannotator-tui config prints colors as the config spells them (light-blue, 238, #rrggbb), and the archive's selected row stays readable with a custom accent.